Vascular endothelial growth factor (VEGF) has been associated with retinal diseases such as AMD, diabetic retinopathy, retinopathy of prematurity, sickle cell retinopathy, retinal vascular occlusion, and inherited retinal dystrophies (Bock et al. 2007; Shams and Ianchulev 2006). The VEGF is a dimeric glycoprotein (approximately 40 kDa), a potent endothelial cell mitogen that stimulates proliferation, migration, and tube formation leading to angiogenic growth of new blood vessels (Bock et al. 2007; Shams and Ianchulev 2006). The VEGF family consists of seven members in mammals, which are VEGF-A (also known as VEGF), VEGF-B to VEGF-F, and PlGF (placental growth factor) (Bock et al. 2007; Shams and Ianchulev 2006). The VEGF receptors belong to the receptor tyrosine kinase family and named as VEGFR-1/Flt-1, VEGFR-2/KDR/Flk-1, Flt-3/Flk-2, and VEGFR-3/Flt-4 receptors (Yancopoulos et al. 2000). The first two receptors, i.e., VEGFR-1/Flt-1 and VEGFR-2/KDR/Flk-1, are primarily involved in angiogenesis, and the latter two VEGF receptors are involved in hematopoiesis and lymphangiogenesis (Yancopoulos et al. 2000).

VEGF expressions were seen in astrocytes of the neuroblastic layer near the optic disc and in Müller cells of the inner nuclear layer (Stone et al. 1995). The expression of VEGF advances towards the periphery with a gradual downregulation in the central retina (Stone et al. 1995). The retinal cells that are involved in the production of VEGF are retinal pigmented epithelium (RPE), astrocytes, Müller cells, vascular endothelium, and ganglion cells (Provis et al. 1997). The gene expression of VEGF is regulated by oxygen tension that helps tissues adjust vascular supply to oxygen demand (Goldberg and Schneider 1994). Animal models involving rats and mice demonstrated that retinal cells produce adequate levels of VEGF, and its expression was decreased in response to hyperoxia thus playing an important role in retinopathy under hypoxic conditions (Alon et al. 1995; Pierce et al. 1996). Additionally, studies under hypoxic conditions have shown Müller cells and astrocytes produce higher amounts of VEGF (Morrison and Aschner 2007). Furthermore, clinical data obtained in humans also suggested role of VEGF in retinopathy and other ocular diseases (Penn et al. 2008). This is supported by studies where levels of VEGF were increased in ocular fluid from patients with diabetic retinopathy and other retinal neovascularizing diseases (Adamis et al. 1994; Pe’er et al. 1995).

Ocular anti-VEGF therapy approach represents one of the significant advances in ocular drug therapy (Penn et al. 2008). Multiple studies in patients have demonstrated the role of antiangiogenic therapies targeting VEGF as an effective therapy in treating AMD (Penn et al. 2008). Globally, AMD is the leading cause of legal blindness, affecting 10–13 % of adults over 65 years of age based on increased life expectancy (Friedman et al. 2004). Additionally, the growing negative impact of environmental risk factors such as arteriosclerosis, obesity, and smoking is expected to double the AMD by 2020 (Friedman et al. 2004). Patients affected by AMD have shown improvements with anti-VEGF treatments (Penn et al. 2008). In addition, the improvement in their vision was enhanced due to increased understanding of the mechanisms of ocular angiogenesis (Schmidt-Erfurth et al. 2014).

Bevacizumab is a recombinant humanized monoclonal antibody that blocks angiogenesis by inhibiting VEGF. It was the first clinically available angiogenesis inhibitor (Rosenfeld et al. 2005). Bevacizumab was approved by the Food and Drug Administration (FDA) in 2004 as anti-VEGF therapy for AMD (Rosenfeld et al. 2005). Moshfeghi et al. studied the effect of systemic bevacizumab therapy on patients with AMD in a 24-week uncontrolled open-label clinical study. The results showed improvement in visual acuity within the first 2 weeks and by 24 weeks (Moshfeghi et al. 2006). Overall, the bevacizumab therapy was well tolerated and effective in patients (Moshfeghi et al. 2006). Another monoclonal antibody fragment (Fab) is ranibizumab, which was created from the same parent mouse antibody as bevacizumab, and the effectiveness is similar to that of bevacizumab (Schmucker et al. 2012). Heier et al. assessed the safety of ranibizumab in treating AMD in a multidose study. Sixty-four patients with subfoveal predominantly or minimally classic AMD-related choroidal neovascularization were enrolled in the study (Heier et al. 2006). The results demonstrated improvement in visual acuity from baseline and decease in areas of leakage and subretinal fluid (Heier et al. 2006). Additionally, repeated intravitreal injections of ranibizumab had a good safety profile in subjects with neovascular AMD (Heier et al. 2006).

Aflibercept is another example that inhibits receptor binding by trapping VEGF in the extracellular space (Stewart 2013). It inhibits the activity of the vascular endothelial growth factor subtypes VEGF-A and VEGF-B thus inhibiting the growth of new blood vessels (Stewart 2013). Zehetner et al. studied the systemic levels of vascular endothelial growth factor before and after intravitreal injection of aflibercept in patients with exudative AMD (Zehetner et al. 2015). Seven days after intravitreal injection of aflibercept, plasma levels were significantly reduced to values below the minimum detectable dose (MDD) in 17 of 19 patients (89.5 %) resulting in a median VEGF concentration of <9 pg/ml (p < 0.001) (Zehetner et al. 2015). The reduction persisted throughout 1 month with values below the MDD in 5 of 19 patients (26.3 %) and a median measurement of 17.0 pg/ml (p < 0.001) (Zehetner et al. 2015). Thus the results demonstrated significant reduction of VEGF levels throughout the observational period of 4 weeks suggesting role of anti-VEGF therapy and effectiveness against AMD (Zehetner et al. 2015). In a recent study, Kawashima et al. looked at the visual and anatomic outcomes in response to the changing treatment to aflibercept in patients with neovascular AMD and polypoidal choroidal vasculopathy (PCV) refractory to previous treatment with ranibizumab. The results suggested switching to aflibercept is generally effective regardless of patient genotype (Kawashima et al. 2014). Comparing between the ocular diseases, PCV patients showed benefit more significantly than AMD patients (Kawashima et al. 2014). In an another study, Michalewski et al. studied efficacy of single dose of aflibercept in bevacizumab nonresponders patients with persistent intraretinal or subretinal fluid treated with 6 or more monthly bevacizumab (Michalewski et al. 2014). After switching to aflibercept, the visual acuity was significantly improved (p = 0.01) and was stable for the remaining 6 months of the study suggesting aflibercept improves visual outcome in bevacizumab nonresponders (Michalewski et al. 2014). Moreover, Peden et al. reported the long-term outcomes in eyes receiving fixed-interval dosing of anti-VEGF therapy in the form of ranibizumab, bevacizumab, or aflibercept administration for at least 5 years in 109 eyes with exudative AMD. The results showed greatest visual gains at 5 and 7 years in those patients with AMD with vision stabilizing or improving in 93.2 % of eyes with anti-VEGF therapy (Peden et al. 2015). Additionally, 43.2 % of patients maintained driving vision in the treatment eye at 7 years compared with 10.1 % at baseline suggesting better outcomes with continuous therapy over sporadic therapy (Peden et al. 2015).

In addition to AMD, other indications such as diabetic macular edema (DME) and proliferative diabetic retinopathy have also been explored to be treated by anti-VEGF therapy (Penn et al. 2008). Diabetic macular edema (DME) is one of the primary causes of visual impairment due to diabetic retinopathy (Penn et al. 2008). The traditional standard treatment of DME is photocoagulation, but with the advent of anti-VEGF therapies, the role of anti-VEGF agents has become increasingly explored (Penn et al. 2008). Adelman et al. compared the efficacy of different ocular drug therapies in the treatment of DME using clinical information provided on 2603 patients with macular edema including 870 patients with DME. The administration of anti-VEGF therapy resulted in significant improvement in visual functions in DME patients (Adelman et al. 2015). The use of anti-VEGF therapy also produced an inhibition of vascular proliferation which resulted into improvement of vision of people with proliferative diabetic retinopathy (Martinez-Zapata et al. 2014). The proliferative diabetic retinopathy is a complication of diabetic retinopathy where panretinal photocoagulation (PRP) is the treatment of choice (Martinez-Zapata et al. 2014). In a study reported by Kambhampati et al., dendrimer triamcinolone acetonide conjugates used as antiangiogenic agent in proliferative diabetic retinopathy showed efficacy by suppressing VEGF production in hypoxic retinal pigment epithelial cells (Kambhampati et al. 2015).

There are various other alternative approaches to inhibit ocular angiogenesis that can be explored during ocular drug discovery and development to improve vision loss related to neovascularization and diabetic retinopathy (Penn et al. 2008; Jacot and Sherris 2011). The PI3K/AKT/MTOR pathway is an intracellular signaling pathway important in regulating the cell cycle. The inhibition of PI3K/AKT/MTOR pathway presents a unique opportunity for the management of ocular neovascularization and diabetic retinopathy (Penn et al. 2008; Jacot and Sherris 2011). The PI3K/AKT/MTOR inhibitors work by suppressing HIF-1α, VEGF, leakage, and breakdown of the blood-retinal barrier thus imparting a pronounced inhibitory effect on inflammation (Penn et al. 2008; Jacot and Sherris 2011). Eventually, the inhibitors also suppress IkappaB kinase (IKK) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) along with downstream inflammatory cytokines, chemokines, and adhesion molecules (Penn et al. 2008). Jacot et al. and Sasore et al. have studied deciphering combinations of PI3K/AKT/mTOR pathway drugs which could result into augmenting antiangiogenic efficacy in vivo (Jacot et al. 2011; Sasore et al. 2014). Using both the zebra fish intersegmental vessel and hyaloid vessel assays to measure the in vivo antiangiogenic efficacy of PI3K/Akt/mTOR pathway inhibitors, the results highlighted the potential of combinations of PI3K/AKT/mTOR pathway inhibitors to safely and effectively treat ocular neovascularization (Sasore et al. 2014). Additionally, some other examples of PI3K/AKT/MTOR inhibitors are everolimus and palomid drugs which are under clinical development in patients with wet AMD (Sasore et al. 2014).

Another potential approach in ocular drug discovery is sphingosine-1-phosphate (S1P), a signaling sphingolipid, also known as lysosphingolipid (Maines et al. 2006). It is an extracellular ligand for S1P receptor 1 and involved in the regulation of a variety of cellular processes. Skoura et al. in the murine model of oxygen-induced retinopathy demonstrated a knockout of S1P2 receptors has been shown to mitigate vascular growth and permeability (Skoura et al. 2007). Preclinical studies have shown intravitreous anti-S1P monoclonal antibody inhibited CNV formation and subretinal collagen deposition (Xie et al. 2009). The mechanism of action of S1P appears to be independent from those of anti-VEGF agents, indicating the potential of S1P antibodies to serve as a monotherapy, or it could be an adjunct therapy to anti-VEGF agents for the treatment of neovascular AMD (Xie et al. 2009).

Glaucoma is a progressive optic neuropathy which results into loss of vision caused by impairment of trabecular meshwork, optic nerve head, and retinal ganglion cells (Sommer et al. 1991). It is also described as a group of eye disorders that result in optic nerve damage, often associated with IOP (Sommer et al. 1991). Glaucoma can be categorized into (a) open-angle and (b) closed-angle glaucoma (Sommer et al. 1991). The open-angle glaucoma is developed slowly over time, is painless, and often has no symptoms until the disease has progressed significantly (Sommer et al. 1991). The treatment of open-angle glaucoma is to lower the pressure or utilize pressure-reducing glaucoma surgeries (Sommer et al. 1991). The other form as closed-angle glaucoma resulting from a sudden spike in intraocular pressure is associated with sudden eye pain, redness, nausea, and vomiting, and it is treated as a medical emergency (Sommer et al. 1991). These two forms of glaucoma have affected 60.5 million people worldwide in 2010, and this number would increase to 79.6 million by 2020 (Resnikoff et al. 2004). Epidemiological studies have shown that African-American with ages 60 and older and people with a family history of glaucoma are susceptible to glaucoma (Resnikoff et al. 2004). Significant advances are being made by pharmaceutical companies and academia for the successful management of glaucoma that include discovery of novel molecular entities and delivery systems to improve medical therapy. Thus, discovering new drugs to lower IOP is another potential area in ocular research.

Intraocular pressure is a condition when the rate of aqueous inflow is the same as the rate of aqueous outflow (Liu and Weinreb 2011). The aqueous humor is drained by trabecular meshwork and the uveoscleral pathway, whereas it is secreted into the eye by the ciliary body (Weinreb 2011). The IOP shows diurnal variation for normal eyes, ranging from 3 to 6 mmHg, and the variation may increase in glaucomatous eyes (Sena and Lindsley 2013). Ocular hypertension is the most important risk factor for glaucoma, and it has been observed that increase in IOP is associated with certain types of glaucoma, as well as iritis or retinal detachment. The IOP can be elevated due to anatomic problems, increasing age, inflammation of the eye, genetic factors, and side effect from coadministered medication (Sena and Lindsley 2013).

Multiple classes of medications are used to treat glaucoma, with several different drugs in each class (Sommer et al. 1991; Resnikoff et al. 2004; Liu et al. 2011). Among the multiple therapeutic agents for lowering IOP in glaucoma, cholinergic agents have been extensively used as they act on muscarinic receptors located on the ciliary muscle to increase outflow through the trabecular meshwork (Migdal 2000). The most common drug that is used under cholinergic agent is pilocarpine; however, its use is limited by the ocular side effect such as miosis, myopia, browache, and dimming of vision (Migdal 2000). Another category of medication is carbonic anhydrase inhibitors that act through suppressing the activity of carbonic anhydrase enzyme (Scozzafava and Supuran 2014). The primary function of carbonic anhydrase enzyme is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues and to help transport carbon dioxide out of tissues (Scozzafava and Supuran 2014). The dorzolamide and brinzolamide are the most commonly utilized carbonic anhydrase inhibitors to lower intraocular pressure in patients with open-angle glaucoma or ocular hypertension (Scozzafava and Supuran 2014; Pinard et al. 2013). Another target to lower IOP is the use of beta-adrenergic receptor blockers (Nathanson 1981). Beta-blockers are a class of drugs that are developed earlier for the management of cardiac arrhythmias; however, they have shown to reduce IOP by reducing aqueous humor formation (Nathanson 1981). They act by blocking adrenergic β-receptor from activation of adenylyl cyclase and cAMP formation to regulate aqueous humor formation in the ciliary process (Nathanson 1981). Timolol, a beta-blocker, has been used to treat open-angle and occasionally secondary glaucoma acts by reducing aqueous humor production through blockage of the beta-receptors on the ciliary epithelium (Daka and Trkulja 2014). Inoue et al. evaluated dorzolamide and timolol fixed-combination eyedrops versus the separate use of both drugs in 34 patients with either primary open-angle glaucoma or ocular hypertension. The replacement of concomitant treatment with dorzolamide or timolol maleate eyedrops with dorzolamide/timolol maleate combination eyedrops improved protocol adherence and preserved the IOP (Inoue et al. 2012).

The alpha-adrenergic receptor, a G protein-coupled receptor (GPCR) agonist, is used in the treatment of glaucoma by decreasing the production of aqueous fluid by the ciliary bodies of the eye and also by increasing uveoscleral outflow (Arthur and Cantor 2011). The most common alpha-adrenergic receptors used clinically are apraclonidine, brimonidine, and dipivefrin (Arthur and Cantor 2011). Fan et al. studied daytime and nighttime effects of brimonidine on IOP and aqueous humor dynamics in thirty participants with ocular hypertension. The results in subjects with ocular hypertension showed brimonidine treatment for 6 weeks significantly reduced seated IOP during the day by increasing uveoscleral outflow (Fan et al. 2014). Chen et al. evaluated the comparison of efficacy and safety of brimonidine with those of apraclonidine in preventing IOP elevations after anterior segment laser surgery in 80 patients (Chen 2005). The results demonstrated a single preoperative drop of brimonidine had similar efficacy and safety as apraclonidine in preventing IOP elevations immediately after anterior segment laser surgery (Chen 2005). Developing prostaglandin F receptor (PTGFR) analogs is another area as these agents have shown to increase aqueous outflow by altering the composition of the extracellular matrix in the ciliary muscle and trabecular meshwork (Lindén and Alm 1999). The analogs for PTGFR have been reported to be the most effective of the IOP-lowering agents; however, adverse effects such as lengthening and thickening of eyelashes have been observed suggesting the need of developing safer PTGFR analogs (Lindén and Alm 1999). The most common analogs are latanoprost, travoprost, bimatoprost, and unoprostone (Lindén and Alm 1999). The PF04217329 is another selective agonist of prostaglandin E receptor 2 in clinical development with studies showing significant reduction of IOP in patients with primary open-angle glaucoma and ocular hypertension (Schachar et al. 2011).

Another potential target in the treatment of glaucoma is developing drugs targeting Rho/ROCK pathway (Wang et al. 2013). It plays an important role in the synthesis of extracellular matrix components in the aqueous humor outflow tissue and the permeability of Schlemm’s canal endothelial cells (Wang et al. 2013). The activation of the Rho/ROCK pathway results in trabecular meshwork (TM) contraction (Wang et al. 2013). In addition, the inhibition of this pathway would provoke relaxation of TM with subsequent increase in outflow facility resulting in decrease of IOP (Wang et al. 2013). The RhoA/ROCK pathway is also involved in optic nerve neuroprotection, improves retinal ganglion cell (RGC) survival, and promotes RGC axon regeneration (Wang et al. 2013). Overall, ROCK inhibitors have the ability to lower IOP in patients with primary open-angle glaucoma and/or ocular hypertension by increasing the outflow through the trabecular meshwork (Wang et al. 2013). Another class of drugs used in glaucoma are macrolides (El Sayed et al. 2006; Rasmussen et al. 2014). These are a group of drugs whose activity stems from the presence of a macrolide ring. The latrunculins are novel marine compounds isolated from the sponge Latrunculia magnifica that have the ability to inhibit actin polymerization thus resulting in increased outflow (El Sayed et al. 2006). Rasmussen et al. evaluated the safety, tolerability, and IOP-lowering effect of latrunculin in patients with ocular hypertension or early primary open-angle glaucoma. In a randomized, placebo-controlled, ascending-dose study in patients, twice daily latrunculin significantly lowered IOP compared with contralateral, placebo-treated eyes, with few and mild ocular adverse events (Rasmussen et al. 2014).

Recently, endothelin system has also been reported to be involved in the processes that lead to vascular dysregulation in glaucoma (Good and Kahook 2010). Endothelin, a vasoconstrictive agent, has shown to be elevated in aqueous humor of patients with glaucoma thus capable of inducing the contraction of both trabecular meshwork cells and the cellular matrix (Good and Kahook 2010). The endothelin receptor would be another target to lower IOP and could constitute a potential new treatment modality to manage glaucoma through IOP reduction. Resch et al. studied the effect of dual endothelin receptor blockade on ocular blood flow in patients with glaucoma and healthy subjects. Bosentan 500 mg daily for 8 days was administered in fourteen patients with primary open-angle glaucoma (Resch et al. 2009). Retinal arteries and veins showed a significant dilatation after administration of bosentan with increased choroidal and optic nerve head blood flow.

Connective tissue growth factor (CTGF) has also been involved in the pathogenesis of glaucoma (Su et al. 2013). The CTGF induces extracellular matrix (ECM) synthesis and contractility in human trabecular meshwork (HTM) cells (Su et al. 2013). Su et al. studied adenovirus-carried connective tissue growth factor on extracellular matrix in trabecular meshwork and its role on aqueous humor outflow facility. The results of the study suggested transfection with adenovirus-CTGF significantly affects the aqueous humor outflow pattern thus demonstrating CTGF is one of the novel targets for treatment of primary open-angle glaucoma (Su et al. 2013). Overall, better knowledge about trabecular outflow pathway, cytoskeletal reorganization, and cell adhesion may be needed in discovering new drug molecules for glaucoma.

The complement system, an innate immunity system of the body, consists of a number of small proteins found in the blood that supports the ability of antibodies and phagocytic cells to clear pathogens from an organism (Jha et al. 2007). Additionally, these plasma and membrane bound proteins play an important role in the defense against infection and in the modulation of immune and inflammatory responses (Jha et al. 2007). Various studies have reported the complement system pathway in the eye (cornea, aqueous humor, tears, and retina) (Jha et al. 2007). In addition, different proteins which are present in ocular tissues that regulate the activation of the complement system are C1 esterase inhibitor (C1 inhibitor), decay-accelerating factor (DAF), membrane cofactor protein (MCP), MAC-inhibitory protein (MAC-IP), factor I, and factor H (Jha et al. 2007). Sohn et al. have reported that complement system works at the level of C3 convertase to prevent the intraocular inflammation (Sohn et al. 2007). The complement system is involved in protecting cornea from pathogenic microorganisms and inflammatory antigens in response to bacterial infection (Mondino et al. 1996).

Both humans and preclinical animal models have shown the involvement of complement pathway in the pathogenesis of a large number of diseases, including ocular diseases (Thurman and Holers 2006). The ocular disease include corneal diseases, autoimmune uveitis, AMD, and diabetic retinopathy. Sivaprasad et al. determined the role of systemic complement activation in the pathogenesis of AMD (Sivaprasad et al. 2007). In 42 subjects each with early age-related maculopathy and neovascular (wet) age-related macular degeneration, plasma complement C3adesArg levels and a single nucleotide polymorphism at position 402 of the complement factor H gene (CFH) were determined (Sivaprasad et al. 2007). The results demonstrated increase in the complement C3adesArg concentration compared to the age-matched controls suggesting C3adesArg is an indicator of complement activation in AMD (Sivaprasad et al. 2007). The complement pathway has also been involved in non-exudative AMD as studies conducted by Johnson et al. have shown the accumulation of abnormal extracellular deposits, called drusen, adjacent to the basal surface of the retinal-pigmented epithelium cells (Johnson et al. 2011). Joachim et al. evaluated the incidence, progression, and associated risk factors of medium drusen in AMD patients. Among 1317 participants at risk, the 15-year cumulative incidence of medium drusen was 13.9 %, and the increasing age and the presence of at least-risk alleles of the CFH were associated with a higher incidence (Joachim et al. 2015). Additionally, the progression rate to late AMD in the eyes with both medium drusen and retinal pigmentary abnormalities was fourfold higher than that in the eyes with medium drusen alone (Joachim et al. 2015).

Under the complement activation family, factor H is another member of the regulators and also called as complement control protein (Józsi and Zipfel 2008). It is a soluble glycoprotein with 155 kDa structure which is involved in the regulation of alternative pathway of the complement system (Józsi and Zipfel 2008). It helps the complement system in directing towards pathogens or other dangerous material and does not damage host tissue (Józsi and Zipfel 2008). In ocular pharmacology, factor H-like proteins regulate the complement system by acting as a cofactor for factor H-mediated inactivation of C3b (Zipfel et al. 2002). In addition, factor H-like proteins are present in vitreous fluid of the eyes and expressed by retinal pigment epithelium cells (Zipfel et al. 2002). A variation in factor H gene is associated with a significant risk in humans with AMD (Hageman et al. 2005). Khandhadia et al. investigated whether modification of liver complement factor H (CFH) production, by alteration of liver CFH Y402H genotype through liver transplantation, influences the development of AMD in patients greater than 55 years (Khandhadia et al. 2013). The results showed AMD was associated with recipient CFH Y402H genotype and local intraocular complement activity is important in AMD pathogenesis (Khandhadia et al. 2013). The involvement of complement system plays an important role in the pathogenesis of diabetic retinopathy (Kastelan et al. 2007). Gerl et al. evaluated the presence of activated complement components in eyes affected by diabetic retinopathy in the eyes of 50 deceased donors with diabetic retinopathy and of 10 nondiabetic subjects with uveal melanoma (Gerl et al. 2002). Utilizing immunohistochemical studies, extensive deposits of complement C5b-9 complexes were detected in the choriocapillaris in diabetic retinopathy (Gerl et al. 2002). The presence of C3d, C5b-9, and vitronectin suggested that complement activation occurs to completion in the eyes affected by diabetic retinopathy.

Complement system has also been involved in uveitis, a disease associated with inflammation of the uvea (Mondino et al. 1984). Under uveitis, anterior uveitis (AU) is the most prevalent form of uveitis, and complement activation products such as C3b and C4b have been demonstrated to be present in the eyes of patients with AU (Mondino et al. 1984). Jha et al. reported complement system plays a critical role in the development of experimental autoimmune anterior uveitis. In an animal model, autoimmune anterior uveitis was induced by immunization with bovine melanin-associated antigen (Jha et al. 2007). The results demonstrated a correlation between ocular complement activation and disease progression in autoimmune anterior uveitis (Jha et al. 2007). Moreover, the incidence and severity of disease were dramatically reduced after active immunization in complement-depleted rats thus suggesting presence of complement was critical for local production of cytokines, chemokines, and adhesion molecules (Jha et al. 2007). The finding suggested complement activation plays a central role in the pathogenesis of ocular autoimmunity and may serve as a potential target for therapeutic intervention (Jha et al. 2007). The complement system has also been involved in protecting cornea from insults due to pathogenic microorganisms and inflammatory antigens (Cleveland et al. 1983). Depletion of complement system protein C3 may result into corneal infection (Cleveland et al. 1983). Therefore, suggesting the functions associated with C3, such as opsonization and regulation of phagocytosis, may be critical in protection of the cornea from bacterial infection. In summary, various strategies in drug discovery can be explored to modulate complement system for discovering new medicine in ocular diseases.

Visual cycle inhibitors act by reducing the accumulation of fluorophores in retinal pigment epithelium cells (Mata et al. 2000; Sparrow et al. 2000). The visual cycle is involved in regenerating 11-cis-retinal by sequence of enzymatic reactions through a pathway located in both retinal pigment epithelium and photoreceptor cells (Mata et al. 2000; Sparrow et al. 2000). This pathway serves as a substrate for the uptake of retinol by the retinal pigment epithelium cells within the eye, whereas aberrant accumulation of cellular debris or lipofuscin results into ocular disease (Mata et al. 2000; Sparrow et al. 2000). The ACU-4429 is a first in class small-molecule visual cycle modulator that inhibits the isomerase complex (Kubota et al. 2012). In mouse model of retinal degeneration, after 45-min dark adaptation, electroretinographic findings demonstrated dose-related slowing of the rate of recovery after administration of ACU-4429 (Kubota et al. 2012). In another study, Moiseyev et al. studied inhibition of visual cycle by A2E through direct interaction with retinal pigment epithelium 65 isomerohydrolase and implications in stargardt disease (Moiseyev et al. 2010). Pyridinium bis-retinoid A2E is a major component of lipofuscin which accumulates in retinal pigment epithelium cells in stargardt disease and contributes to the disease pathogenesis (Moiseyev et al. 2010). The study demonstrated A2E efficiently inhibits with retinal pigment epithelium 65 isomerohydrolase enzyme (Moiseyev et al. 2010). Furthermore, the experiments demonstrated the fluorescence of retinal pigment epithelium 65 isomerohydrolase decreased upon incubation with A2E suggesting A2E inhibits the regeneration of 11-cis retinal, a mechanism by which A2E may impair vision in stargardt disease (Moiseyev et al. 2010).

Other visual cycle inhibitor that is under clinical development is fenretinide, an oral synthetic retinoid derivative (Berni and Formelli 1992). Under physiological conditions, the retinol necessary for the regeneration of 11-cis-retinal is delivered to the retinal pigment cells in a complex formed by retinol-binding protein (RBP), retinol, and transthyretin (TTR) (Berni and Formelli 1992). Fenretinide works through binding to the RBP in the circulation and prevents the association with retinol thus inhibiting the transport of retinol to the RPE and decreasing its presence in the visual cycle (Berni and Formelli 1992). Additionally, RBP-fenretinide complex is removed by the kidneys through urine, thus reducing the circulating quantity of RBP (Berni and Formelli 1992; Mata et al. 2013). Samuel et al. studied the effect of fenretinide in inducing ubiquitin-dependent proteasomal degradation of stearoyl-CoA desaturase in human retinal pigment epithelial cells (Samuel et al. 2014). The study demonstrated fenretinide decreased stearoyl-CoA desaturase protein and enzymatic activity suggesting role of fenretinide against retinal diseases (Samuel et al. 2014). In a study with 246 patients for treatment of geographic atrophy in AMD, the efficacy of fenretinide was studied after oral administration of 100 and 300 mg dosing for 2 years (Mata et al. 2013). The results demonstrated dose-dependent reversible reductions in serum RBP-retinol, reduced lesion growth rates, and reduced rate of choroidal neovascularization (Mata et al. 2013). In summary, discovering compounds to be visual cycle inhibitors is a promising area in ocular research, and the discovery of novel compounds can benefit various ocular diseases.

With increasing knowledge of vitreoretinal disorders and the role of anomalous posterior vitreous detachment in vitreomaculopathies, pharmacologic vitreolysis has emerged as a new treatment modality (Sebag 1998). Pharmacologic vitreolysis is a nonsurgical approach to release tractional forces at the vitreoretinal interface by injecting an enzyme with proteolytic activity against fibronectin and laminin into the vitreous cavity (Sebag 1998). Sebag firstly defined pharmacologic vitreolytic compounds as agents that alter the molecular organization of vitreous in an effort to reduce or eliminate its role in disease (Sebag 1998). The agents used as pharmacologic vitreolysis can be classified by their mechanism of action, whether they induce liquefaction of the vitreous (liquefactant) or vitreous separation from the retina (interfactant) (Bandello et al. 2013). The tissue plasminogen activator (tPA), plasmin, microplasmin, nattokinase, and vitreosolve are believed to be both liquefactants and interfactants, whereas hyaluronidase is used as liquefactant. The majority of agents used for pharmacologic vitreolysis are enzymes (tPA, microplasmin, nattokinase, chondroitinase, dispase, and hyaluronidase), whereas the nonenzymatic agents used are urea/vitreosolve and arginine-glycine-aspartate peptides (Bandello et al. 2013).

Plasmin, a nonspecific serine protease, acts by degrading fibrin and other extracellular matrix components, including laminin and fibronectin (Liotta et al. 1981). Plasmin may also indirectly generate increased levels of other nonspecific proteases such as matrix metalloproteinases and elastase (Baramova et al. 1997). Both preclinical and clinical studies have shown ability of plasmin to achieve a complete vitreoretinal separation (Liotta et al. 1981). In addition, strong correlation was observed between plasmin concentration, exposure time, and the resultant extent of vitreoretinal separation (Liotta et al. 1981). Plasmin as autologous plasmin enzyme (APE) was used in the surgical treatment of pediatric traumatic macular holes (Margherio et al. 1998), stage 5 retinopathy of prematurity (Wu et al. 2008), and complicated X-linked retinoschisis showing improved anatomic outcomes. Ocriplasmin (microplasmin) is a recombinant product which has proteolytic activity of targeting vitreoretinal interface such as fibronectin and laminin (Tsui et al. 2012). The mechanism of ocriplasmin involves two steps, i.e., it is involved in vitreoretinal separation and vitreous liquefaction (Tsui et al. 2012). Ocriplasmin has shown greater penetration of vitreous and epiretinal tissues as compared to plasmin with reduced risk of microbial contamination associated with blood derivatives (Tsui et al. 2012). Multiple studies in animal models have shown its activity using porcine, rat, and rabbit eyes (Tsui et al. 2012). Ocriplasmin demonstrated efficacy and safety for the treatment of patients with symptomatic vitreomacular adhesion or vitreomacular traction, including patients with macular holes (Khoshnevis and Sebag 2015). The patients treated with ocriplasmin were more likely not to require vitrectomy surgery (Khoshnevis and Sebag 2015). In a clinical trial with 125 patients who underwent pars plana vitrectomy for the treatment of either vitreomacular traction or macular hole, the results suggested ocriplasmin injection at a dose of 125 μg led to a greater likelihood of induction and progression of posterior vitreous detachment than placebo injection (Khoshnevis and Sebag 2015). The other pharmacologic vitreolysis agent chondroitinase acts by degrading chondroitin sulfate, whereas nattokinase has a strong fibrinolytic affect (Bandello et al. 2013). Dispase is a protease which cleaves fibronectin, collagen IV, and, to a lesser extent, collagen I, and hyaluronidase acts by dissolving the glycosaminoglycan network of the vitreous gel thus electively considered a liquefactant (Bandello et al. 2013).